"For 150 years, these cells have been known to exist in the brain, but we haven't known exactly what they are doing in adults," says Dr. Berislav Zlokovic, the neuroscientist leading the research. "It turns out that pericytes are very important for helping maintain a brain environment crucial to the health of neurons."

Pericytes wrap around capillaries in the brain like ivy wrapping around a pipe, to help maintain the structural integrity of the vessels. They're also central to determining the amount of blood flowing in the brain and in maintaining the barrier that stops toxic substances from leaking out of the capillaries and into brain tissue.

When the scientists reduced the number of pericytes in research mice, they saw a reduced blood flow, greater exposure of brain tissue to toxic substances, impaired learning and memory, and damage to the neurons all of which are more likely to happen to people as they age. Compared to normal mice, the mice with fewer pericytes had structural damage to their neurons, with the older mice having more damage than the younger mice. Researchers also documented impaired learning and memory in the middle-age and older mice, but not the youngest mice.

"Our findings show that chronic vascular damage due to pericyte loss results in neurodegeneration," Zlokovic says. "It may be that a vascular insult is common to many different types of neurodegenerative processes and may be significant in causing the symptoms seen in diseases such as Alzheimer's and amyotrophic lateral sclerosis."